ment 368 (2006) 79–87www.elsevier.com/locate/scitotenv

Science of the Total Environ

Mercury content in vegetation and soils of the Almadénmining area (Spain)

R. Millán a,⁎, R. Gamarra b, T. Schmid a, M.J. Sierra a, A.J. Quejido a, D.M. Sánchez a,A.I. Cardona a, M. Fernández a, R. Vera a

a CIEMAT, Avda. Complutense 22, E-28040 Madrid, Spainb Universidad Autónoma de Madrid, Dpto. de Biología, E-28049 Madrid, Spain

Received 15 October 2004; received in revised form 12 May 2005; accepted 29 September 2005Available online 15 December 2005

Abstract

The Almadén area is an exceptional case of mercury concentration in the world, where cinnabar is the predominant mineralisedform. Mining activities have been carried out over the past two thousand years and once this activity is concluded, a restoration ofthe environment is necessary for the affected areas. That implies the selection, optimisation and application of the most appropriatetechniques focused on the recovery of contaminated areas. Phytotechnology is one of the proposed techniques, which considers theuse of plants due to the low environmental impact and their proved efficiency. In this framework, it is necessary to study thebehaviour of mercury in the soil–plant system and to determine the mercury absorption capacity of the different Almadén plantspecies. Therefore, field and laboratory studies have been carried out to determine the easily available forms of mercury in ten testplots in the Almadén area and the resulting data is related to the mercury accumulation in different plant species of the studiedterritory. Special attention is given to the easily available forms of mercury in the analysed soils and used as the transfer factor forthe Hg-availability index that relates the amount of mercury in a plant with the total amount of mercury in a soil and with the easilyavailable forms. The result of this experimental work is selecting the plant species suitable to be used in a future implementation ofphytotechnologies in this area.© 2005 Elsevier B.V. All rights reserved.

Keywords: Almadén; Vegetation; Mercury; Phytotechnology; Hg-availability

1. Introduction

Almadén is the world's largest mercury mining dis-trict and is located in Central Spain within the Provinceof Ciudad Real (Autonomous Region of Castilla LaMancha), extending over an area of about 100 km2.Production from Almadén represents more than 30%(285,000 t) of the total known mercury producedthroughout the world (Hernández et al., 1999). Mining

⁎ Corresponding author. Tel.: +34 91 3466704; fax: +34 91 3466269.E-mail address: rocio.millan@ciemat.es (R. Millán).

0048-9697/$ - see front matter © 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.scitotenv.2005.09.096

activity at the Almadén mine ceased in May 2002, butsignificant quantities of ore were stockpiled prior to thattime and retorting of the ore was discontinued at Alma-dén in February 2004 (Newman, 2002; Gray et al.,2004). Hg ore at Almadén is dominantly cinnabar(HgS), but elemental Hg (Hg0) is also present and insome deposits it is locally abundant. The three mostimportant mines included: Almadén, El Entredicho (anopen pit) and Las Cuevas. As a result of the Hg oreretorting process, most Hg compounds were convertedto Hg0. A significant part of this Hg0 was lost to thesurrounding environment or remained in mine wastes

80 R. Millán et al. / Science of the Total Environment 368 (2006) 79–87

that were commonly discarded on site. As much as 25%of the total Hg recovered from some Hg mines waspossibly lost, contaminating surrounding environments(Hines et al., 2000). Even after many years of inactivity,areas of Hg mining contain elevated Hg concentrationsprimarily as a result of inefficient mining proceduresand incomplete extraction of Hg, and Hg contaminationcan be significant over 100 km from the source. Inaddition, highly reactive, water-soluble ionic Hg saltssuch as calomel (Hg2Cl2) and mercury oxychlorides(i.e., Hg2ClO or Hg4Cl2O), which are probably formedduring Hg extraction, have been observed in Hg minewastes. Due to the large size of the Almadén miningdistrict, Hg emissions and downstream dispersion haspotentially contaminated a significant part of the sur-rounding environment. Nowadays, relatively littleenvironmental research has been carried out at Almadén(Lindqvist and Rodhe, 1985; Adriano, 2001; Gray et al.,2004).

The climate for the Almadén region is defined asMediterranean with wet and cool winters, and dry warmsummers. The Almadén mining district is morphologi-cally characterized by WNW trending narrow valleys(∼5 km wide) and mountains, within a landscape ran-ging in altitude between 200 and 1000 m above sealevel. This basin and range type realm is geologicallycontrolled by the Hercynian Central Iberian tectonicWNW trend and the conspicuous outcrops of theArmorican Quartzite (Ordovician). Other equivalentlithological units of younger age (e.g., Criadero Quart-zite; Lower Silurian) define the crests of the surround-ing hills. The Paleozoic sequence comprisesepicontinental quartz arenite rocks, including blackshales and quartzites. Ordovician to Devonian magma-tism accounts for diatremes, alkaline lava of differentcomposition, and late tholeiitic diabases (Higueras etal., 2000a,b).

Studies by Lindberg et al. (1979) report that a totalmercury concentration in the soil surface was found inthe range of 0.5–260 μg g−1; where maximum valueswere obtained near the mine. Significant mercury con-centrations in soils were found at distances of up to 14km in the direction of the prevailing winds. More recentstudies carried out in the Almadenejos metallurgical siteshow that soil surfaces display the most scattered set ofvalues (6–8889 mg kg−1). Other sampling areas aroundChillón, Rodoviejo, and the Castilseras site are in theranges of 12–132 mg kg−1, 10–188 mg kg−1 and 6–69mg kg−1 Hg, respectively. These are expected valuesfor a region rich in mercury deposits and subjected to anintense and prolonged mining activity (Higueras et al.,2004; Gray et al., 2004). Further data shows mercury

concentration values that ranges from 5 to 1710 mgkg−1. Minimum values were found in the experimentalfields corresponding to the agricultural areas. Mediumvalues corresponded to areas located near Almadén andin the direction of the prevailing winds (12–122 mgkg−1). Maximum values were found in the proximity ofEl Entredicho open pit, and also, close to Las CuevasAlmadén mines (140–1750 mg kg−1) (Millán et al.,2003a, 2004). Besides the anthropogenic contamina-tion, the submarine hydrothermal activity during Silur-ian and Devonian time has created a naturalmetallogenic enrichment of mercury in the district(Higueras et al., 2000b).

Data from Hildebrand et al. (1980) showed mer-cury concentrations in plants greater than 100 μg g−1

and 1 μg g−1 within 0.5 km and at 20 km distantfrom the mine, respectively. A field campaign per-formed by Maserati et al. (1996) found the maximumvalue in plant in Spartium junceum (20 μg g−1),collected inside the mine complex. Recent studiescarried out in experimental plots of the Almadénmining area showed that mercury concentration inplants ranged from below the detection limits togreater than 1000 mg kg−1 (Millán et al., 2003a;Millán et al., 2004; Higueras et al., 2004).

In order that a contaminant is assimilated, it hasto be mobilised and transported and be available tothe organism. In this work, it is assumed that thesoluble and the more easily extracted mercury cor-responds to the Hg-available fraction for plants. Ageneric definition of bioavailability is “the potentialof living organisms to take up chemicals from thefood or from the abiotic environment to the extentthat the chemicals may become involved in themetabolism of the organism”. Therefore, bioavail-ability is the key factor (driver) defining the extentand magnitude of biological responses in the dose–response relationships in terrestrial and aquatic eco-systems (Adriano, 2001).

Chemical extraction techniques are still the mostcommonly used method for estimating the fractionof a micronutrient or contaminant that is availablein the short term. The most widely used methodsfor the evaluation of the availability of metals insoils are single extraction and sequential extractionmethods.

Several single extraction methods are used for theevaluation of the phytoavailability of metals in soils(metals extracted with CaCl2, DTPA, acetic acid,water, etc.). A systematic comparison, however, islacking. In the sequential extraction methods the num-ber of phases determined varies typically from four to

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as many as six to seven per sample. In general, themethod involves the selection of chemical reagentsfrom the least to the most aggressive in a sequentialfashion and from the least to the greatest extremes intemperature and stirring. There are prescribed extrac-tion procedures for the specific soil/sediment fractionssuch as soluble, exchangeable, sorbed, organic, oxide-crystalline or amorphous Fe and Mn oxides, carbo-nate, sulfide and residual (Adriano, 2001; Wang et al.,2004, Sánchez et al., 2005).

In general, plants contain low quantities of mercuryin their cells, given that this metal is toxic and does nothave any known biological function (Beauford et al.,1977). Certain plants are capable of absorbing mercury,which is present in an available form in the soil (Mal-ikova, 1999). The root acts as a barrier against mercuryentering the intracellular ways. This is due to a forma-tion of ligands between the metal and carboxyl groups(Siegel et al., 1987).

The translocation of the mercury to aerial parts of theplants is a very common process in those species cap-able of accumulating mercury. The transport pathway isthrough the xylem and through a flow regulated fortranspiration such as the phloem. The latter way enablesthe metal to reach all parts of the plant including fruitsand seeds (Lucena et al., 1993).

In this work, field and laboratory studies have beencarried out to determine the easily available mercury inten plots in the Almadén area and these data are relatedto the mercury accumulation in different plant speciesof the Almadén flora.

2. Material and methods

2.1. Description of plots

Soils vary from poorly to well developed on slopesand in the bottom of valleys, respectively. According tothe Soil Taxonomy (Soil Survey Staff, 1999) they areclassed as Entisols, Inceptisols or Alfisols with a xericmoisture regime (Schmid et al., 2003). The evidence ofsoil disturbances by human activities such as mining isoften encountered. The affected soils are thereforeregarded as anthropogenic soils (Wilding and Ahrens,2001). From the botanical point of view, the Almadénarea is classed into the Mediterranean Region (Luso-Extremadurense Subprovince). The flora, in theMesomediterranean belt, is characterised by dominantsclerophyllous formations (Mediterranean oaks andcork oaks) on siliceous bedrock. In the study area, theclimax community is defined as Pyro bourgaeanae–Quercetum rotundifoliae. The floristic composition

shows a main presence of hemicryptophytes, came-phytes and nanophanerophytes typical of the Mediter-ranean environment (Millán et al., 2003b).

In this research project a total of ten experimental testplots have been selected in the Almadén area accordingto the botanical classification of the vegetation in thearea. These plots (Fig. 1), of approximately 1 Ha, arelocated in mining sites and under the influence of air-borne emissions from mining activities around theAlmadén, El Entredicho and Las Cuevas mines, and inthe Almadenejos village and its surroundings. Fieldcampaigns were carried out to determine the character-istics for the following test plots.

2.1.1. Plot 1—Valdeazogues riverPlant communities along the river are dominated

by Nerium oleander and Flueggea tinctoria. Alongthe course of the river there appear sporadic smallpopulations of Typha domingensis or Phragmites aus-tralis, which through their rhizomes extend graduallytaking advantage of open spaces after the eliminationof other hygrophytes. The degradation of the originalcommunity is also due to the culture of small popula-tions of Eucalyptus camaldulensis. [Plant studied: N.oleander L.].

2.1.2. Plot 2—Source of JardinilloOn the slopes of the hills close to remains of

siliceous Mediterranean oak (P. bourgaeanae–Q.rotundifoliae), there are a scrub community belongingto the phytosociological association of Genisto hirsu-tae–Cistetum ladaniferi. These thickets mainly consistof a dense scrub represented by several species of thegenus Cistus (Cistus ladanifer, Cistus monspeliensis,Cistus crispus, Cistus salviifolius and Cistus albidus),accompanied by other bushes and shrubs typical of theclimatic formation of the Mediterranean oak. [Plantsstudied: C. albidus L., C. crispus L., C. ladanifer L.,C. monspeliensis L., C. salviifolius L.].

2.1.3. Plot 3—South facing slope of Sierra deCordoneros

The prairie contains the characteristic meadowswith Hordeion alliance under the influence of cattleand between enclosures mainly formed of Quercusilex subsp. ballota. The meadows are made up ofdifferent species and their dominance changes duringthe springtime. These include a great diversity ofhemicryptophytes and some therophytes with an ele-vated representation of individuals. [Plants studied:Erodium cicutarium (L.) L'Hér., Geranium molle L.,Galactites tomentosa Moench].

Fig. 1. The location of the test plots (P1 to P10) within the study area of Almadén.

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2.1.4. Plot 4—Almadenejos, road to GargantielA grassland is present containing Mediterranean oak

and the evidence of agricultural activities, which havebeen abandoned and therefore shows a succession ofspecies associated to more recent events. Under morehumid conditions, a community dominated by the Pha-laris coerulescens is present with other species of thefamily Poaceae such as Aegilops triuncialis, Aegilopsventricosa. In less humid and more ruderal conditionsEruca vesicaria, Diplotaxis virgata, appear with Chro-zophora obliqua. One part of this plot was used to plantEucalyptus camadulensis. [Plant studied: E. vesicaria(L.) Cav.].

2.1.5. Plot 5—Almadenejos villageThe plot is within an abandoned old metallurgy plant

from the 17th and 18th centuries. Nowadays this plot iscovered with cinnabar mine tailings and submitted to thepressure of pig farming. The area is mainly covered byMarrubium vulgare plants sharing the territory with

other nitrophilous species such as Urtica urens, Cyno-glossum cheirifolium and D. virgata, a nitrophilousassociation typical of the alliance Onopordion nervosii.[Plants studied: M. vulgare L., C. cheirifolium L.].

2.1.6. Plot 6—Almadén mineSteep slopes are dominated by Rumex induratus,

which stabilize the soil due to their root system andtherefore, reducing erosion. Only in areas with a morefixed substrate, there are siliceous species such as C.crispus and Cytisus scoparius. In the zones recently dis-turbed, only therophytes such as Rumex bucephalopho-rus are present. The area is under strong pressure oflivestock. [Plant studied: R. induratus Boiss. andReuter].

2.1.7. Plot 7—South facing slope of Sierra deCordoneros

The alteration of the evergreen oak forest hasresulted in pastures and individual shrubs of Retama

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sphaerocarpa. The shrub favours the soil stability andnutrition. These pasturelands are dominated by thero-phytes and hemicryptophytes of a low height and witha heliophytic character such as Spergularia rubra,Plantago lagopus, Chamaemelum mixtum, Diplotaxiscatholica and Anacyclus clavatus. [Plants studied: R.sphaerocarpa (L.) Boiss., Scolymus hispanicus L., A.clavatus (Desf.) Pers.].

2.1.8. Plot 8—El Entredicho mine, upper talusOn the older talus, marked by material initially

extracted from the mine, there are remains of the P.bourgaeanae–Q. rotundifoliae Rivas Martínez 1987community, together with sclerophytic shrubs contain-ing Pistacia lentiscus, Quercus coccifera, C. ladaniferand C. monspeliensis. In open areas with rock outcropsdifferent hemicryptophytes such as Erysimum lagascaeappear. [Plant analysed: E. lagascae Rivas Goday andBellot].

2.1.9. Plot 9—El Entredicho mine, lower talusSpecies such as Bituminaria bituminosa, R. indura-

tus and Dittrichia viscosa are colonizing the lowest partof the mine and the steepest slopes of the open pit. Inareas with accumulating or exuding water, small forma-tions of T. domingensis and Tamarix africana are found.[Plants analysed: B. bituminosa (L.) Stirton, R. indur-atus Boiss. and Reuter, C. ladanifer L.].

2.1.10. Plot 10—Las Cuevas mineAn open Mediterranean oak forest belonging to the

association P. bourgaeanae–Q. rotundifoliae RivasMartínez 1987, with vast formation of C. ladanifer,accompanied with C. monspeliensis and Cytisus stria-tus. [Plants analysed: C. monspeliensis L., Asparagusacutifolius L. (young turions), C. ladanifer L.].

2.2. Vegetation sampling and analysis

The vegetation sampling was carried out in the testplots during June 2002 and June 2003. Each selectedplant species was sampled ten times. An exception wasplot 1 where only 6 samples were selected. Samplingconsisted of obtaining the stems and leaves for shrubsand the entire plant for therophytes. The location of theindividual samples was registered with a portable Glo-bal Positioning System (GPS). Individual samples wereidentified in the field and brought in paper bags to thelaboratory for their preparation within 24 h and taxo-nomic classification.

Vegetative structures (stem and leaves) were used fordetermining the mercury content. These samples were

placed in individual beakers and rinsed several timeswith distilled water and cleaned using an ultrasonic bathto remove external contamination. All samples wereoven dried at 35 °C for 48 h, then they were groundand sieved to a size less than 50 μm.

2.3. Soil sampling and analysis

Soil samples were taken at the location of eachindividual plant by introducing a metal cylinder, withdimensions of 25 cm depth and 20 cm diameter intothe soil surface. The corresponding content of thecylinder at each sampling point was filled into aplastic storage bag and transported to the laboratoryfor soil analyses.

Soil samples were sieved and air dried. The fineearth fraction (b2 mm) was further oven dried at 35°C for 48 h. Thereafter, the fine earth fraction washomogenized and ground with an agate mortar andsieved with a mesh to obtain a particle size of lessthan 420 μm. The two first steps of a seven-stepsequential extraction procedure (Quejido et al., 2003;Sánchez et al., 2003) were applied to each soil samplefrom the different plots selected in the Almadén area.Five hundred milligrams were processed by triplicateusing a seven-step sequential extraction protocol(SEP). The sample and the reagent weres mixed inpropylene tubes and after the reaction conditions wereapplied, the mixture was centrifuged. Thereafter, thesupernatant was removed for analysis and the residue,after washing with de-ionised water, was treated withthe next reagent. The SEP includes step 1 (hydroso-luble salts) where the samples were treated with 25 mlof de-ionised water at pH 7.0 for 1 h at room tem-perature. In step 2 (exchangeable cations) the residuewas mixed with 20 ml of 1 M ammonium chloride atpH 7.0 for 1 h at room temperature (Quejido et al.,2004; Sánchez et al., 2005).

Mercury content in all the samples of soil andvegetation and in all the extracts of the SEP wasmeasured using an Advanced Mercury Analyser(AMA-254, LECO Company). Five replicates ofeach sample were measured and the results areexpressed as the mean value within two standarddeviations. Certified reference materials (CRM) wereused to determine the accuracy and precision of themeasurements and validate the applied methods. Theywere obtained from the Community Bureau of Refer-ence (BCR). These reference materials are denomi-nated BCR-CRM 62 (olive leaves, 0.28±0.02 mgkg−1 of Hg) and BCR-CRM 281 (rye-grass, 0.0205±0.0019 mg kg−1 of Hg) and the average value for

Table 1Mercury content in soil samples (total, soluble and exchangeable Hg)

Testplot

Total Hg(mg kg−1)

Soluble Hg(mg kg−1)

ExchangeableHg (mg kg−1)

Available Hg a

(mg kg−1)

P1 146±13 0.0227±0.0022 0.360±0.026 0.383±0.026P2 5.03±0.43 0.0417±0.0037 0.285±0.019 0.327±0.019P3 12.8±1.2 0.0323±0.0017 0.274±0.024 0.306±0.024P4 21.3±1.1 0.0850±0.0055 0.281±0.024 0.366±0.024P5 550±58 1.040±0.033 4.4±2.3 5.4±2.3P6 1710±43 0.609±0.013 7.3±1.3 7.9±1.3P7 5.53±0.49 0.0453±0.0058 0.182±0.012 0.227±0.013P8 69.8±6.8 0.033±0.021 0.254±0.008 0.287±0.022P9 122.4±7.5 b0.02 1.20±0.10 1.20±0.10P10 118±12 0.0210±0.0025 0.149±0.052 0.169±0.052a Available Hg=Soluble Hg+Exchangeable Hg.

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ten measurements was 0.290±0.002 mg kg−1 and0.0210±0.0003 mg kg−1, respectively.

3. Results and discussion

The toxicity and environmental mobility of mer-cury compounds is closely related to their associationwith different mineral phases. Cinnabar and other mer-cury sulphides, mercury oxides, metallic mercury andorgano-mercury compounds are the most commonmercury forms found near mercury mining sites. How-ever, some clay materials and oxyhydroxides of sometransition metals have been described as sinks of inor-ganic mercury.

Table 1 displays the content of total mercury in soil,and the soluble and exchangeable mercury from thesamples of the ten test plots considered in this work.The easily available mercury is considered the sum ofthe soluble and exchangeable forms. The mercury asso-ciated to water-soluble phases and exchangeable forms

Fig. 2. The mercury as soluble (Hgs), exchangeable (Hgex) and availab

was very low, taking into account the high content ofmercury in these soils. A comparison of the percentagesof soluble, exchangeable and available forms of mer-cury with respect to the total mercury content in the soilis shown in Fig. 2.

Total mercury content in soils ranges from 5 to 1710mg kg−1. The higher values have been found in the plots6N5N1N9N10. All of them exceed 100 mg kg−1, andwhere the plots 5 and 6 have the highest values of 550 and1710 mg kg−1, respectively. These field plots are withinthe close vicinity of the mining areas, where cinnabarextraction and treatment have taken place. Therefore,the soils from these plots are regarded as anthropogenicas a result of the mining activities. Plot 1 on the banks ofthe Valdeazogues River lies along the rail tracks and roadwhich was used to transport the mineral ore. The analyseson the soils from the remaining plots show lower totalmercury concentrations. These plots are located at differ-ent distances from areas containing the highest mercuryconcentration, and their land uses are mainly pasture-lands, forest and shrub areas.

The soluble mercury content ranges from b0.02 to1.04 mg kg−1. The higher concentrations were found inthe old metallurgy plant (plot 5) and in the mine tailingsof the Almadén mine (plot 6) with values of 1.040 and0.609 mg kg−1, respectively. The values for the differentplots are very low in comparison with the total mercurycontent. In all cases the soluble mercury in the soil solu-tion is b1% with respect to the total concentration of thisheavy metal. The highest percentage of mercury is 0.8%and corresponds to plots 2 and 7 with a total mercurycontent of 5.03 and 5.53 mg kg−1, respectively.

The concentrations of exchangeable Hg are in therange between 0.15 and 7.3 mg kg−1. The major con-centrations of mercury found in the exchangeable

le (Hga) forms relative to the total mercury (Hgt) content in soil.

Table 2Mercury content in natural vegetation and their corresponding transferfactors

Testplot

Plant species Mercury contentin aerial part(mg kg−1)

TFt a TFa b

P1 Neriumoleander

0.63±0.04 0.0043±0.0005 1.64±0.15

P2 Cistussalvifolius

0.52±0.03 0.103±0.011 1.59±0.13

Asparagusacutifolius

0.67±0.06 0.133±0.016 2.05±0.22

Cistusladanifer

0.79±0.02 0.157±0.014 2.42±0.15

Cistusmonspeliensis

0.87±0.07 0.173±0.020 2.66±0.27

Cistus crispus 1.18±0.12 0.235±0.031 3.61±0.42Cistus albidus 1.29±0.10 0.256±0.030 3.94±0.38

P3 Asphodelusalbus

0.45±0.04 0.035±0.005 1.47±0.17

Geraniummolle

0.80±0.02 0.063±0.006 2.62±0.21

Erodiumcicutarium

1.14±0.07 0.089±0.010 3.73±0.37

Cirsium sp 1.20±0.07 0.094±0.011 3.92±0.38P4 Eruca

vesicaria2.12±0.21 0.099±0.011 5.80±0.69

P5 Marrubiumvulgare

38.5±3.2 0.070±0.009 7.1±3.1

Cynoglossumcheirifolium

43.48±0.21 0.079±0.008 8.1±3.4

P6 Rumexinduratus

22.7±2.2 0.0133±0.0003 2.88±0.47

P7 Retamasphaerocarpa

0.58±0.01 0.105±0.009 2.56±0.15

Matricariarecutita

1.57±0.12 0.284±0.033 6.92±0.66

Scolmbushispanicus

2.72±0.29 0.492±0.068 12.0±1.5

P8 Erysimunlagascae

12.51±0.73 0.179±0.020 43.5±4.2

P9 Bituminariabituminosa

4.43±0.33 0.036±0.003 3.66±0.42

Cistusladanifer

5.12±0.29 0.042±0.003 4.24±0.44

Rumexinduratus

15.48±1.23 0.126±0.013 12.8±1.5

Tamarixafricana

43.7±4.0 0.357±0.039 36.1±4.5

P10 Cistusmonspeliensis

1.84±0.13 0.016±0.002 10.9±3.4

Asparagusacutifolius

2.03±0.06 0.017±0.002 12.0±3.7

Cistusladanifer

2.83±0.19 0.024±0.003 16.7±5.2

a TFt= [Hg] plant/[Hg]total soil.b Tfa (Hg-available transfer factor)= [Hg] plant/[Hg]available soil.

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fraction refers to the plots 6, 5 and 9 with values of 7.3,4.4 and 1.2 mg kg−1, respectively. Relative to the totalmercury in the soil, the major percentage of mercuryfound in the exchangeable fraction is 5.67% and corre-sponds to plot 2.

The plots with the highest Hg concentrations (6, 5and 9) are soils under anthropogenic influence. Theseareas are influenced by mining and metallurgy andcoincide with the most easily available mercury. Thesoils found in the remaining plots with pastures andnatural vegetation does not follow the above tendency.This shows that other soil parameters and properties thatinfluence the solubility and availability of mercury willbe studied in future research work.

Nowadays, there is a common conception that thetotal concentrations of metals in soils are not a goodindicator of phytoavailability, or a good tool for poten-tial risk assessment, due to the different and complexdistribution patterns of metals among various chemicalspecies or solid phases. Experimentally defined chemi-cal procedures such as mercury thermo-speciation(Mashyanov et al., 2004), speciation (Lifvergren andAllard, 2004) and sequential extraction (Kocman andHorvat, 2004; Sánchez et al., 2005) have been appliedto identify plant-available forms of trace metals in soils.However, apart from available easily available forms,the mercury is mainly adsorbed in soil by clay, hydro-xides and organic matter or in a precipitated form(Adriano, 2001).

It has long been recognized that the soluble,exchangeable and loosely adsorbed metals are quitelabile, and hence more available for plants. Therefore,in order to assess the environmental risk and the phy-toavailability of metals, efforts should be concentratedon the measurements of these available fractions (Wanget al., 2004).

The analysis of vegetation on contaminated land orin mining areas may be utilised as a toxicity index foridentifying the accumulation of heavy metals and toprovide an insight into their mobilization sequencesand availability (Banásová, 1999; Murphy et al.,2000). In this case, the soil to plant Transfer Factor(TF) can be obtained by dividing the mercury concen-tration in the plant by the mercury concentration in thesoil. This work presents two different TF according tothe mercury concentration fraction considered (Table 2).The first one corresponds to the total mercury in the soil(TFt), and the second one takes into account the easilyavailable mercury content (TFa).

A general tendency shows that plots with higher totalmercury content in soil leads to the highest concentra-tion in plants (P5, P6 and P9). An exception is the case

of A. acutifolius, where the TFt value decreases whenthe total mercury in soil increases and the TFa increases.R. induratus present in field plots 6 and 9, showed a

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different behaviour with respect to the TFb. This time,an increase in the total mercury concentration in the soilleads to a decrease of the TFb.

Another general tendency found in the studiedplant species is that higher available mercury valuesin the soil implies a higher content in the plant. Thissuggests that mercury translocation inside the plants,from the roots to the aerial part, has taken place. Theplants with a high mercury contents are located inareas with elevated total mercury concentrations insoil, due to anthropogenic activities such as mining.Although, available mercury is low, a process such asmercury volatilisation that leads to the incorporationof mercury vapour via the stomata has to be takeninto account.

The data show that annual and biannual plantshave lower mercury contents, compared to pluriann-ual plants, such as dwarf shrubs and bushes, thatpresent higher contents (T. africana). These variationsin the content are related to the biomass and thebiotype of the plants and are conditions to be takeninto account.

These results show a complexity of metal iondynamics in the soil system and the interactive role ofplant and environmental factors on the whole process.The total mercury concentration is not a definitive datawhen determining the transfer to the vegetation, as thisprocess depends on the available fraction of the con-taminant in the soil. Therefore, differences in the mer-cury concentrations in the soils of the plots as well as inthe plants indicate that other determining factors in thesoil processes and the plant transfer have to be takeninto account.

Research is now in progress to determine the soilparameters that influence the mercury availability aswell as the processes that control the mercury uptake inplants. This work has clearly shown that different spe-cies within a plot have a wide range of mercury contentin the plants. Furthermore, the same species have adifferent behaviour in the mercury uptake from differentplots that will be included in future research work.

4. Conclusions

The Almadén area shows a great variability in mer-cury content in soil, but this mercury is mainly in a non-available form. However, the plant species show a dif-ferent capability to absorb and translocate mercury.Plants from plot 4 are annual and biannual species,and present less mercury content than the other plots.There is similar mercury content in plants that belong tothe same botanical family.

Plants from plots where mercury content in soil ishigher show a high capability to accumulate mercury inthe aerial part. All the selected species, M. vulgare, C.cheirifolium, R. induratus and T. africana, are perennialplants (vivaceous herbs and scrubs), which couldincrease the period for mercury to be accumulated instems and leaves. All of them could be considered asphytoextractors to be used in phytotechnology.

The implemented sequential extraction protocol willbe useful to provide information about mercury avail-able in different contaminated areas. Very low availablemercury, including water-soluble and exchangeablefractions, was found in all samples.

The total mercury content is frequently used to assessthe extent of the contamination accumulated and toprovide an overview of the long term potential toxicityeffects. However, the determination of total mercuryconcentrations is not sufficient for an understanding ofthe environmental impact and pathways of the differentspecies. Transport, availability, and physicochemicaland toxicological properties are highly dependent onthe chemical form that is present in the environment.

Acknowledgements

This work is financed by the Spanish Ministry ofEducation and Science (Project reference: REN 2002-04229-C02-TECNO).

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